The temperature in the vicinity of the top ring of a gasoline engine reaches a high temperature equal to or greater than 200° C. due to combustion of fuel. Under such a high-temperature in the internal combustion engine, combustion pressure causes the piston ring and the piston ring groove surface of the piston (hereinafter referred to as the “ring groove surface”) to repeatedly collide with each other, while at the same time, the piston ring surface and the ring groove surface slide in the circumferential direction.
FIG. 1 shows the results of measurements of the roughness of the ring groove surface. On the ring groove surface, projections of about 1 μm in height, which have been caused by lathing using cutting tools, exist at intervals of 0.2 mm. To reduce the weight of a vehicle body, it is a typical practice to employ an aluminum alloy as the piston material; however, an aluminum-based material has a low heat resistance and a reduced hardness at above 200° C. When the ring groove surface collides with and slides relative to the piston ring under such a high temperature, fatigue failure may occur thereon for the projections on the surface to drop off, resulting a new surface of an active aluminum alloy appearing on the ring groove surface. Then, the dropped aluminum alloy pieces or the new aluminum alloy surface that has appeared within the ring groove may collide with the piston ring and thereby be brought into contact with the upper and lower faces of the piston ring, being further subjected to sliding. This leads to the “aluminum cohesion” which causes the aluminum alloy pieces to adhere to the side faces of the piston ring or the piston ring body to adhere to the new aluminum alloy surface of the piston. The aluminum cohesion continues to occur so long as the new aluminum alloy surface is produced, so that the progression of the aluminum cohesion would cause the piston ring to fixedly adhere to the piston within the ring groove, compromising the sealing performances of the piston ring. Losing one of the sealing performances or the gas sealing function would lead to the blowby phenomenon in which a high-pressure combustion gas flows out of the combustion chamber into the crankcase, resulting in a drop in engine output. Furthermore, losing the oil sealing function would lead to an increase in oil consumption. Furthermore, the aluminum cohesion would cause ring groove wear to progress and thereby degrade the sealability between the upper or lower face of the piston ring and the ring groove surface, leading to an increase in the amount of blowby.
To prevent the aluminum cohesion, a number of methods have conventionally been suggested for allowing the aluminum alloy, which is the base material of the piston, and a piston ring, particularly, the top ring not to be directly brought into contact with each other, or for alleviating the attack to the ring groove by the piston ring.
As the piston-side countermeasures, disclosed in Patent Literature 1 is a method for performing anodic oxidation treatment (or alumite treatment) on the ring groove surface and then filling in the fine holes, which have been produced by the processing, with a lubricating substance. Since the alumite treatment serves to form a hard film mainly composed of aluminum oxide on the ring groove surface, the aluminum alloy as the piston base material is prevented from dropping off and thus from adhering to the piston ring. However, the anodic oxidation treatment on the piston requires high costs and the resulting hard aluminum oxide provides bad initial running-in properties.
On the other hand, as the countermeasures on the piston ring side, for example, disclosed in Patent Literature 2 is a method for forming a film on the side face of the piston ring, the film having a solid lubricant such as molybdenum disulfide dispersed in a heat-resistant resin such as polyamide or polyimide. In the structure described in Patent Literature 2, the solid lubricant in the film is cleaved and worn to reduce the friction coefficient of the film, thereby alleviating the attack to the ring groove and preventing the aluminum cohesion. Furthermore, disclosed in Patent Literature 3 is a method for forming the surface film made of a heat-resistant resin containing copper-based powder on the side faces of the piston ring. In Patent Literature 3, it is stated that the addition of the copper-based powder can impart wear resistance to the surface film formed on the piston ring surfaces, thus allowing the heat-resistant resin to provide the lubricating function for a long time.
Furthermore, it is stated in Patent Literature 4 that a coat layer, which is mainly composed of a polyamide-imide resin and comprises a polyamide-imide resin film reforming agent and a dry film lubricant containing hard particles such as alumina, is formed on the sliding surface of a sliding member having streaks of a predetermined surface roughness, thereby reducing the friction coefficient of the sliding member while providing improved wear resistance and contact properties thereto. It is also stated that from the viewpoint of balance between the wear resistance and the wear of a counterpart member, alumina or silicon nitride that has a predetermined hardness is preferably employed as the hard particles.
Recently, an increasingly high temperature is reached in the vicinity of the top ring as the engine provides higher output power. Under this situation, fatigue failure tends to more easily occur due to degradation in piston strength, making it more difficult to maintain the resin film coated on the piston ring for a long time. In Patent Literature 2, the solid lubricant is added as an indispensable component; however, as described above, the solid lubricant itself is cleaved and worn to reduce the friction coefficient of the film, thus alleviating the attack to the ring groove. For this reason, the film has a low wear resistance, so that it is difficult to maintain the film for a long time so as to sustain the effect of preventing the aluminum cohesion. Furthermore, retarding the wear of such a film would restrict the amount of added solid lubricant as well as the reduction of the friction coefficient of the film. Thus, there is a possibility that the surface of the piston material having been reduced in hardness under high temperatures will be roughened, further causing the occurrence of aluminum cohesion.
Furthermore, even the films disclosed in Patent Literatures 3 and 4 cannot sufficiently reduce the attack to the counterpart member, so that there will be a possibility that under high temperatures, the surface of the counterpart member is roughened and the film itself disappears due to wear or thermal decomposition.
As can be seen from above, there has been no piston ring available which can maintain the effect of highly preventing aluminum cohesion for a long time in a high output engine.